A 72-year-old retired soldier has blown himself up in a park in the Japanese city of Utsunomiya, killing himself and injuring three other people in an apparent suicide.

A second explosion, separate from the blast that killed the former Self Defence Force member, caused a fire in a nearby parking lot, while the man's home 8 km from the park burnt to the ground, state broadcaster NHK reported.

The police in the city, which is about 100 km north of Tokyo, said they had discovered a suicide note written by the man and were investigating the three incidents, but did not give further details.

Images broadcast by NHK showed two burnt out cars, one of which belonged to the dead man, about 200 metres from the park and a charred broken bench at the edge of the park, where a folk art festival was underway.

The explosion in the park injured two men aged 64 and 58, who were being treated for shrapnel wounds.

The third victim was a 14 year-old school boy, NHK said.

Witnesses told the broadcaster they heard a series of loud explosions.

Suicide rates have declined in Japan in recent years, but remain among the highest in the world, with around 30,000 people a year taking their own lives.

Experts have pointed to the financial stress of surviving on small pensions for pushing some retired people to end their lives.

In 2015, a 71 year-old man set himself on fire on a bullet train in an incident that also claimed the life of a 52-year-old woman travelling in the same carriage.

Are you affected by an issue in this article? Here's where to get help:

Lifeline - 0800 543 354
Suicide Crisis Helpline (open 24/7) - 0508 828 865 (0508 TAUTOKO)
Depression Helpline - 0800 111 757
Samaritans - 0800 726 666
Youthline (open 24/7) - 0800 376 633. Text 234 for free between 8am and midnight, or email [email protected].

0800 WHATSUP (0800 9428 787), Open between 1pm and 10pm on weekdays and from 3pm to 10pm on weekends. Online chat is available from 7pm to 10pm every day at www.whatsup.co.nz.

Healthline - 0800 611 116
For more information about support and services available to you, contact the Mental Health Foundation's free Resource and Information Service on 09 623 4812 during office hours or email [email protected]

If it is an emergency and you feel like you or someone else is at risk, call 111.